WATERLIME GROUP. 39S 



ceous feet behind the swimming feet of Limulus ; and, instead of being 

 double, is anchylosed as in young Limulus. 



Since ray comparisons, during investigations of these fossils, had been 

 made almost entirely with Limulus, I am prepared to appreciate these 

 views of Professor Agassiz*. 



The following is the description of the genus by Dr. Dekat : 



•' Caput a thorace non distinctum. Os ignotum. Oculi duo, sessiles, 

 distantes, lunati. Abdomen elongatum, posticam versus extremitatem 

 sensim gracilius, segmentis transversis subimbricatis divisum. Pedes 

 octo; duo utrinque antici branchiferi, duo utrinque postici maximi, 

 omnes lamellosi." 



The specimens examined enable me to extend this description, and the 

 genus may be characterized as follows : 



r 



GeKIS ElRYPTERlS (Dekay), 



Body ovato-lanceolate, broader in front, gradually attenuated behind, and 

 terminated in a spiniform tail. Carapace on the upper side entire, 

 somewhat semioval, wider than long. Eyes two, reniform or oval, dis* 

 tant, sessile, placed within the margin of the carapace : simple oculi- 

 form tubercles, or corneas, two, situated subcentrally on the carapace. 

 Thoracic and caudal portions composed of thirteen joints ; the first set 

 of these, belonging to the thorax, are narrower ; the others, constituting 

 the caudal or abdominal portion, are subquadrate, except the terminal 

 joint, which is prolonged into a sublinear or lanceolate triangular spine 

 with serrated edges. 

 The first thoracic articulation on the lower side is double, consisting of 



the first two joints of the back anchylosed in one, and showing the suture. 



A central locomotive appendage is attached to and proceeds from the 



upper part of the first joint ; and dividing the lower half of the same, it 



• This volume having been in print for a long time, with tlie exception of the hiatus left for the Eury- 

 pttrut, its completion can no longer be postponed; but the descriptions with the illustrations will appear as 

 • separate paper, for which Prof. Agassiz has promised me a preliminary or introductory note. 



